The present invention relates to an improved process and apparatus for molding plastic articles from particulate thermoplastic material.
There are various techniques for molding plastic articles known in the art, each of which has its own particular advantages and disadvantages. The most common of these techniques is injection molding, wherein liquid plastic is injected into a mold and subsequently cooled. While injection molding is the least expensive process available for forming plastic articles in large quantity, the high cost of producing an injection mold makes its use for low volume work prohibitively expensive. In addition, the injection mold must be quickly cooled to permit its repetitive use at high volume, and that rapid cooling creates stresses within the plastic article that may result in cracks.
Another technique for producing molded plastic articles is called thermoforming. In this process, a plastic sheet is heated and stretched onto a mold. While thermoforming is exceptionally useful for shallow articles, plastic articles of any substantial depth made by that process exhibit internal stresses and weaknesses which may result in cracks. In addition, the sheet plastic used as the raw material is somewhat more expensive than the pellets or powder used in injection molding.
A still further technique for producing molded plastic articles is known as rotational molding. In this process, a fixed charge of plastic is placed within a female mold, and the mold is rotated. During rotation, heat is applied to the outside of the mold to cause the plastic to melt. With this process, energy use is relatively high and the mold is expensive to produce. In addition, the plastic article, although smooth on the outside which contacts the mold, may have a relatively rough surface on the inside. Such a rough surface is a disadvantage when the article is to be used as a container.
The present invention is an improvement of the molding process and apparatus described in the U.S. Pat. No. 4,431,397, granted Feb. 14, 1984, the disclosure of which is incorporated herein by reference. According to this patent, plastic molded articles are formed by surrounding a hollow heat-conductive mold with particulate thermoplastic material. Heat is applied to the interior of the mold until the temperature of the outer surface is above the fusion temperature of the particulate thermoplastic material. This temperature is maintained until an article of desired thickness is formed. Thereafter, loose particulate material is removed from the outer surface of the molded article and heat is again applied to the interior of the mold so as to cause the outer surface of the molded article to become smoother. Finally, the article and mold are cooled and the article is removed from the mold. This molding process will hereinafter be referred to as UNIFUSE(trademark) molding.
It is an object of the present invention to provide an improved process and apparatus for forming plastic molded articles for which the tooling cost for molds is lower than for other plastic molding processes known in the art.
It is an object of the present invention to provide an improved process and apparatus for forming plastic molded articles which uses multiple processing stations in order to speed up the molding process and to increase the throughput per unit time of molding operation.
It is a further object of the present invention to provide an improved process and apparatus for forming molded plastic articles which requires a minimum of capital expenditure for a complete molding production system.
It is a further object of the present invention to provide an improved process and apparatus for forming molded plastic articles which permit the wall thickness of each article to be controlled within fine tolerances.
It is a further object of the present invention to provide an improved process and apparatus for forming molded plastic articles such that the articles exhibit very little internal stress.
It is a further object of the present invention to provide an improved process and apparatus for forming very large, relatively stress-free molded plastic articles such as whole vessel or vehicle bodies or substantial parts thereof.
These objects, as well as other objects which will become apparent from the discussion that follows, are achieved, according to the present invention, by means of an improved UNIFUSE(trademark) process for forming one or more molded articles from particulate thermoplastic material, which process comprises the steps of:
(a) providing a track having a plurality of process stations,
(b) providing a mold assembly comprising a base, at least one hollow mold upstanding from the base, and guide means on the base for moving the mold assembly on the track,
(c) preheating the mold assembly at a preheat station on the track,
(d) moving the mold assembly from the preheat station to a transfer station on the track,
(e) transferring the mold assembly from the transfer station to a molding station,
(f) forming the molded article(s) by fusing particulate material to the hollow mold at the molding station,
(g) transferring the mold assembly with the molded article(s) thereon from the molding station back to the transfer station,
(h) moving the mold assembly from the transfer station to a cooling station on the track,
(i) moving the mold assembly from the cooling station to an unloading station on the track,
(j) removing the molded article(s) from the hollow mold at the unloading station, and
(k) moving the mold assembly from the unloading station to the preheat station again.
This process lends itself to an extremely low tooling cost for each different article to be formed. The heat conductive mold may be formed of sheet material which is welded together and sanded smooth. Aircraft quality aluminum, or aluminum with no pinholes, is the preferred material although other heat-conductive materials may also be used for the mold.
Because of the single surface molds and zero pressure used in the process of the invention, tools for the process can be made more rapidly and at lower cost than for any other molding process. Also, because most molds are made out of aluminum sheet metal, molds can be changed or modified by laser cutting out a section and welding in a new section.
The process of the invention does not require draft or tapered molds. The taper that is essential in other processes is also helpful for easier ejection, but is not essential to the process. Female molds shrink away allowing ejection. For male molds the draft parts can be ejected mechanically or by air. For air ejection, a poppet value is inserted in the mold, which can be connected to an air line. Air pressure of approximately 2 lbs. per square inch will normally slightly bubble the top of the part and the shrinkage onto the male mold sides prevents the air from escaping. When the linear shrinkage on the sides breaks the friction, the part lifts off the mold.
Because no stresses appear in the material as the article is forming, internal stresses do not develop unless the article is cooled quickly. The wall thickness of the molded article formed by this process may be precisely controlled by controlling the time during which heat is applied to fuse the particulate thermoplastic material together.
Further, articles of practically any desired size may be produced, from small cups or corrugated sheet to large panels or sections of motor vehicle bodies.
Finally, the cost of the apparatus according to the invention for carrying out the process is extremely modest in comparison with the capital investment required to purchase apparatus to carry out other known plastic molding processes.
For a better understanding of the invention, together with other and further objects, reference is made to the following description, taken in conjunction with the accompanying drawings.